1. Field of the Invention
The present invention relates to gas fluidized bed reactors. More particularly, though not exclusively, the present invention relates to an apparatus and method for the improved construction of a gas distribution system for a fluidized bed reactor having two reaction zones.
2. Problems in the Art
Several processes have been proposed in the prior art for treating solid particles in a two-zone fluidized bed reactor. For some applications of a two-zone fluidized bed reactor, oxidizing conditions are maintained in one zone while reducing conditions are maintained in another zone so that as the particles circulate in the fluidized bed, they are alternately oxidized and reduced.
One example where such a reactor can be used is for the conversion of calcium sulfide to calcium oxide at a temperature in the range of 900.degree. C. to 1200.degree. C. This example is explained in detail in U.S. Pat. Nos. 5,433,939 and 5,653,955 which are incorporated by reference herein. When calcium sulfide particles are treated with air in this temperature range, an outer layer of each calcium sulfide particle is converted to calcium sulfate which prevents further reaction. However, if the particles are treated subsequently with a reducing gas, the layer of calcium sulfate formed on the surface is converted to calcium oxide, which is porous enough to allow oxygen to penetrate and to react with another layer of calcium sulfide. As the particles are exposed repeatedly to oxidizing and reducing conditions, they are converted one small layer at a time, first to calcium sulfate and then to calcium oxide until all of the calcium sulfide is gone.
Another process where a two-zone fluidized bed reactor can be used is for the conversion of calcium sulfate particles to calcium oxide particles in the 1000.degree. C. to 1200.degree. C. temperature range. This example is explained in detail in U.S. Pat. No. 4,102,989 which is incorporated by reference herein. While reducing conditions are needed to convert calcium sulfate into calcium oxide in this temperature range, and the rate of conversion is proportional to the reducing gas concentration, such conditions also favor side reactions which convert calcium sulfate into calcium sulfide. By using the two-zone fluidized bed reactor in which the circulating particles are alternately and repeatedly exposed to reduction and oxidation, any calcium sulfide produced during a pass through the reducing zone is eliminated during a subsequent pass through the oxidizing zone. After numerous passes through the oxidizing and reducing zones, the particles are converted almost entirely into calcium oxide.
The process of alternately oxidizing and reducing particles has been demonstrated in the prior art in bench-scale two-zone fluidized bed reactors up to ten inches in diameter. This is explained in detail in C. E. Morris, T. D. Wheelock, and L. L. Smith, "Processing Waste Gypsum in a Two-Zone Fluidized Bed Reactor", AIChE Symposium Series No. 255, (Vol. 83), pp. 94-104 (1987), which is incorporated by reference herein. In these prior art reactors, a mixture of primary air and natural gas has been introduced through a refractory grid plate at the bottom of the fluidized bed and excess secondary air has been introduced through an axial ceramic tube which discharged air from above at approximately mid-level in the fluidized bed. By employing a sub-stoichiometric amount of primary air, highly reducing conditions have been created in approximately the lower half of the fluidized bed, and by introducing excess secondary air at mid-level, oxidizing conditions have been created in approximately the upper half of the fluidized bed. Since sulfur dioxide has been released within the fluidized bed, the gas phase has been extremely corrosive for metals at the high reaction temperature. Therefore, the internal components of reactors have been made entirely of refractory materials.
It is very difficult to apply the concept of two-zone fluidized bed reactors on a large commercial scale since the reactor has to be constructed almost entirely of refractory materials in order to withstand the high temperature and corrosive conditions. The design and fabrication of large refractory grid plates for gas distribution and for the support of high temperature fluidized beds is well known in the art. However, existing designs do not provide for the distribution of a second gas higher in the fluidized bed. Therefore, a practical and effective system is needed for distributing different gases at two or more different levels within the same fluidized bed for large scale reactors.